But 21 Jump Street isn’t one of them. Maybe that’s because it’s been turned into a comedy. And a pretty darn funny one, at that.

No one’s been clamoring for a movie version of this property. But now that we’ve got one, we’re better for it.

The original TV series was a crime drama about youthful undercover cops that ran from 1987 to 1991 and introduced soon-to-be-a-teen-idol Johnny Depp to the viewing public. But although the series was nothing if not earnest, the movie version of 21 Jump Street comes to the movie screen as a raucous action romp.

Jonah Hill and Channing Tatum star as Morton Schmidt and Greg Jenko, two young, bored police officers who have bonded at the police academy, Schmidt helping Jenko get through the written tests and Jenko helping Schmidt get through the vigorous physical training.

Now they’re looking for excitement beyond their mundane duties, most of which call for them to be on bike patrol at the local park, looking for errant frisbees.

When they botch a drug-bust arrest, they’re beckoned by one Capt. Dickson, played by Ice Cube, who reassigns them to a long-dormant police program, one which calls for them to go on undercover duty back at their high school, where they are to pose as currently enrolled brothers/students and bust a drug ring led by a dealer played by Dave Franco (James’ younger brother), who is dispensing a dangerous new synthetic drug that has already claimed at least one young life.

When they were in high school seven years back, they were at different places in the social hierarchy. Jenko was a hunky, dumb jock, Schmidt a doughy, bullied nerd.

When they’re assigned new identities, the intent is for Schmidt to go among the outcasts and Jenko among the cool kids. But events conspire to force them to switch identities, thus giving each a chance to see how the other half lived back in the day. Schmidt ends up performing in tights in drama class while Jenko is crouching over a Bunsen burner in AP chemistry lab.

But as they soon discover, high school circa 2012 is significantly different than it was way back in 2005. What was looked down upon then is cool now, and vice versa.

Directors Phil Lord and Chris Miller (the animated Cloudy with a Chance of Meatballs) just about ignore the nostalgia element — wisely, it turns out — and instead invest their energy in sending up the high school comedy as well as the bromantic cop thriller.

But they more or less give up on themselves and their material in the late going, nearly abandoning their comic ambitions and instead resorting to the obligatory car chases, gunfire exchanges, and incendiary explosions.

However, there have been so many big laughs on the way to the flameout climax, we’re willing to wait patiently for them to regain their comic footing in the late going. Which they do.

Hill and Michael Bacall co-wrote the script, which is stimulatingly silly throughout, sometimes laugh-out-loud funny, and only occasionally juvenile, but is self-aware enough to reference Hollywood’s penchant for recycling decades-old, nostalgia-laden material like THIS project rather than carving out new artistic territory.

As an actor, Hill is dependably funny throughout, but it’s the usually solemn Tatum who is a comedic revelation, matching Hill every joke and gag of the way. And because the two leads also served as executive producers, they get at least some of the credit for not only their inspired pairing but seeming to have this much fun performing together without doing self-indulgent damage to the finished product.

And as for the inevitable cameo fans of the original series are craving — well, let’s just advise patience, which will eventually be rewarded. Our lips are otherwise sealed.

So we’ll go undercover as 2½ stars out of 4. Folks who jump onto 21 Jump Street are in for a good time.